One development in modern fighter aircraft is a tendency to increase the installed power. Indeed, manoeuvrability requirements necessitate the provision of highly powered aircraft. The search to reduce the infra-red radiation of aircraft without reducing performance leads to an increase in installed power without after-burn. This is also a consequence of the search for versatility, such as in STOL/VTOL applications.
The outcome, for a given technological development level, is an increase of engine mass which imposes a reduction of overall fuel capacity of up to 20 to 25% in order to keep constant the total mass of the aircraft on take-off.
In the known engines used nowadays, of the bypass turbojet type, suppressing the after-burn makes it possible to obtain, when operating at full throttle, a substantial reduction in fuel consumption which may exceed 100%, but, at constant cruising thrust, the specific fuel consumption increases with the size of engine.
A representation is given in FIG. 1 of the attached drawings, of a curve showing the variation of the ratio Cs/Csm (as ordinate) of the specific consumption at partial throttle and full throttle as a function of the ratio F/Fm (as abscissa) of the corresponding thrusts at partial throttle and maximum throttle. On this curve, point A corresponds to an engine equipped with after-burn, and point B to an engine without after-burn supplying the same thrust on take-off, and therefore of greater size. It will be observed that for an increase of engine size enabling an increase of thrust .DELTA.F/F of 50% to be obtained, the corresponding increase of specific consumption when cruising .DELTA.Cs/Cs is 26.5%.
Indeed, from the analyses carried out the following requirements appear difficult to reconcile, namely:
to obtain a powerful thrust at full throttle, it is necessary to operate with a low bypass ratio and to use an engine with a large high pressure body;
whereas, to obtain a low fuel consumption when cruising, it is necessary to operate with a high bypass ratio and to use an engine with a small high pressure body.